Bypassable wilkinson divider

ABSTRACT

A power divider/combiner which can be installed flexibly, with small changes either as a divider/combiner or as a lossless transmission line. The configuration of the power divider/combiner into a lossless transmission line is realized by a parallel connection of two non-symmetrical transmission lines which usually have different impedances. One of the transmission lines is a branch present in a Wilkinson divider, and the other is an extra branch formed inside the divider/combiner.

This application is the national phase of international application PCT/FI96/ 00325 filed May, 31, 1996 which designated the U.S.

This application is the national phase of international application PCT/FI96/ 00325 filed May, 31, 1996 which designated the U.S.

BACKGROUND OF THE INVENTION

The invention relates to high-frequency engineering, more exactly topower dividers used in microwave and radio engineering.

On high frequencies, especially in microwave and radio engineering, itis often necessary to split a signal into two or more output ports or tocombine several signals into one output port. In some solutions the sameswitching device has to be used either as a power divider from one inputport into two output ports or as a lossless transmission line from oneinput port into one output port as required at each time. This isconventionally implemented by selection devices, such as bridges, placedon circuit boards. For example, a surface-mounted resistor of zero ohmscan operate as a bridging component suitable for industrial massproduction. Standard junction lines (also conventionally called bondwires) can also be used.

One generally used passive switching device is a so-called Wilkinsondivider. The operation of a standard Wilkinson divider appears from FIG.1A. The figure shows a situation in which the signal splits from oneinput port into two output ports. With respect to the present invention,the divider can be used also in the opposite way for combining a signalfrom two input ports into one output port.

When operating as a power divider, the Wilkinson divider comprises aninput port IN, output ports OUTI and OUT2, a T-junction 1, atransmission line 2 connecting the input port IN and the output portOUT1, and a transmission line 3 connecting the input port IN and theoutput port OUT2. The output ports OUT1 and OUT2 are further connectedby a resistor R. The length of the transmission lines is a quarter ofwavelength.

The characteristic impedance of the input port IN is Z₀. Thecharacteristic impedances of the output ports OUTI and OUT2 are Z₁ andZ₂, respectively. In a simple case, when Z₀ =Z₁ =Z₂, the characteristicimpedance of the transmission lines is Z₀ √2 and the impedance of theresistor R is 2Z₀.

In a general case, when Z₀ =Z₁ =Z₂ does not necessarily hold true, thecharacteristic impedance of the transmission line 2 is √2Z₀ Z₁ and,correspondingly, the characteristic impedance of the transmission line 3is √2Z₀ Z₂ The impedance of the resistor R is then 2√Z₁ Z₂ .

A known arrangement for transforming the Wilkinson divider into alossless transmission line is disclosed in FIGS. 1A, 1B, 2A and 2B. Thecircuit in FIG. 1B comprises a transmission line 5 with respect to FIG.1A and bridging devices B1 to B5. FIG. 2A shows how the Wilkinsondivider thus transformed is transformed into a Wilkinson divideraccording to FIG. 1A. In this case, the resistor R and the bridges B1,B4 and B5 are installed, but not the bridges B2 and B3. The transmissionline 5 has in this case no effect on the operation of the divider.

It is shown in FIG. 2B how the Wilkinson divider is bypassed, that is,transformed into a lossless transmission line. In this case, theresistor R is not installed, nor the bridges B1, B4 and B5. When onlythe bridges B2 and B3 are installed, the circuit shown in FIG. 2B is alossless transmission line between the input port IN and the output portOUT1.

A disadvantage of the circuit according to FIG. 1B is e.g., the greatnumber (five in this embodiment) of bridging places operating asselection devices and the great number of installed bridges (three individer use, two as a transmission path). A further disadvantage of theprior art circuit is that the bridges B2 and B3 required for operatingas a transmission path cannot be easily produced with small strayimpedances since they combine wide lines. Another disadvantage of theprior art circuit becomes evident when the input port IN and the outputports OUTI and OUT2 are not opposite to one another, especially when theWilkinson divider is folded to reduce its size. Especially in cases suchas this, it is difficult or even impossible to fit a wide transmissionline within the divider. Furthermore, the arrangement cannot be used atall when the divider is simultaneously being used as an impedanceadapter, that is, Z₁ ≠Z₀.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a power divider whichcan be installed flexibly, with small changes either as a divider or asa lossless transmission line and which does not share the problemsassociated with the prior art arrangement described above. The object isachieved with the arrangement according to the characterizing part of anarrangement in which the configuration of the power divider into alossless transmission line is realized by a parallel connection of twonon-symmetrical transmission lines which usually have differentimpedances. One of the transmission lines is a branch present in theWilkinson divider and the other is an extra branch formed inside thedivider.

BRIEF DESRIPTION OF THE DRAWINGS

The preferred embodiment of the invention is explained further in thefollowing with reference to the attached drawings in which:

FIG. 1A shows a standard Wilkinson divider;

FIG. 1B shows a prior art way of transforming the Wilkinson divider intoa lossless transmission path;

FIG. 2A illustrates a switching device shown in FIG. 1B installed as aWilkinson divider;

FIG. 2B illustrates a switching device shown in FIG. 1B installed as alossless transmission path;

FIG. 3A shows a modified Wilkinson divider according to the invention;

FIG. 3B shows a modified Wilkinson divider according to the inventionfolded into as small a space as possible;

FIG. 3C shows how a coupling device can be folded so that itstransmission lines are not situated in the same place.

FIG. 4A shows a switching device according to the invention installed asa Wilkinson divider;

FIG. 4B shows a switching device according to the invention installed asa lossless transmission path.

DETAILED DESCRIPTION

The solution according to the invention is shown in FIG. 3A. It isassumed herein that Z₁ =Z₀, but the circuit operates in the same way ifZ₁ ≠Z₀.

The idea of the invention is to implement a transmission line with acharacteristic impedance Z₀ by a parallel (i.e., an electricallyparallel) connection of two narrow high-impedance transmission lines:one transmission line 2 with an impedance Z₀√ 2, which is alreadypresent in a standard Wilkinson divider, and another transmission line 4with an impedance 2Z₀ /(2-√2). When Z₀₌ 50Ω) the impedance of thetransmission line 2 should be about 70Ω and the impedance of thetransmission line 4 about 170Ω. The latter impedance cannot be producedon most substrates without special procedures. One such procedure is toetch ground plane from under the 170Ω line 4. Another way is to placethe 170Ω line 4 very close to the 70Ω line 2, whereby the interactionbetween the lines 2 and 4 will raise the impedance of the line 4. Itwould not be very harmful if the impedance were not exactly at itsoptimum value. For example, on a 1.6 mm FR-4 substrate or a 0.76 mmTeflon® polythelyne terephlate substrate, the maximum obtainablecharacteristic impedance is between 140 and 150Ω. With this impedancethe standing wave ratio (VSWR) will be about 1.1.

The operation of the invention is further examined on the basis of FIG.4A. Only the bridge B1 and the resistor R are installed for thesplitting operation. As the bridges B2 and B3 are not present, the bothbranches 2 and 3 of the Wilkinson divider are passages of the signal.The circuit operates now as a standard Wilkinson divider.

Non-splitting operation is studied in FIG. 4B. The bridge B1 and theresistor R are not installed but the bridges B2 and B3 are installed. Inthis case, the signal meets the parallel connection of the transmissionlines 2 and 4 of a quarter of wavelength, the impedances of which areZ_(0/)√ 2 and 2Z₀ /(2-√2), respectively. A quarter-wavelength longtransmission line with the impedance Z₀ is produced by the parallelconnection of the impedances.

FIG. 3B shows how a modified Wilkinson divider according to theinvention can be folded in order to minimize the space it takes up on acircuit board.

FIG 3C shows how a coupling device can be folded so that itstransmission lines are not situated on the same plane. In FIG. 3C, onlytransmission line 3 is shown, comprising a section 3 which has beendrawn with dotted lines and which is located on a different plane thanthe rest of transmission line 3.

An advantage of the solution according to the invention is that theWilkinson divider on the same circuit board can be used as required ateach time both in splitting and non-splitting operation which willreduce the required number of different circuit boards. Also, in thesolution according to the invention a smaller number of bridges andplaces for bridges are needed than in prior art solutions.

A further advantage of the solution according to the invention is thatvery little stray impedance is produced as bridges are needed only inhigh-impedance lines. Another advantage is that the extra line needed inthe Wilkinson divider can easily be fitted into a limited space sincethe extra line is very narrow. The extra line 4 may also run close tothe branch 2 in the Wilkinson divider, as long as the connection istaken into consideration in planning. See e.g. Matthaei, Young andJones, Microwave fllters, impedance-matching networks and couplingstructures, Artech House Books, 1980, Figure 5.09-1, p. 219. By means ofmeandering, a quarter-wavelength long line can be placed into theavailable space.

In the arrangement according to the invention, a power divider, such asa Wilkinson divider, can be configured so that the same componentsubstrate, such as a circuit board, can be used either as a powerdivider from one input port into two output ports or as a losslesstransmission path from one input port into one output port. The changesin the way of operation cause less alterations in the circuit than inconventional solutions.

It is evident to those skilled in the art that the art according to theinvention can be used in conjunction with other transmission lines, suchas microstrips, suspended substrate microstrips, striplines, coaxiallines, coplanar waveguides or combinations of the above mentioned. Theproduction of transmission lines and bridging devices is not restrictedto the example described above, but the field of the invention can varywithin the scope of the claims.

I claim:
 1. A high frequency bypassable power divider/combiner,comprising:a first port, a second port and a third port; a firstquarter-wavelength transmission line, connected between said first portand said second port; a second quarter-wavelength transmission line,arranged to selectively connect said first port to said third port; athird quarter-wavelength transmission line; first, second and thirdinstallable bridging devices selectively located with respect to saidtransmission lines so that only said first and second bridging devicesor said third bridging devices are operable at any one time; a firstlocation comprising said first installable bridging device and a secondlocation comprising said second installable bridging device, said firstand second bridging devices connecting said second transmission linebetween said first port and said second port, and said second portresistively to said third port, said third transmission line remainingunconnected; and third locations comprising said third installablebridging devices, said third bridging devices connecting said thirdtransmission line electrically in parallel with said first transmissionline, said second transmission line remaining unconnected.
 2. The powerdivider/combiner according to claim 1, wherein:said first port has afirst characteristic impedance Z₀, said second port has a secondcharacteristic impedance Z₁, and said third port has a thirdcharacteristic impedance Z₂, and said third quarter-wavelengthtransmission line has a characteristic impedance which is dimensioned sothat the impedance produced by parallel connection of said transmissionline and said third transmission line is substantially equal to √Z₀ Z₁ .3. The power divider/combiner according to claim 1, wherein:said totalis four.
 4. A power divider/combiner according to claim 2, wherein:saidfirst bridging device is a resistor the resistance of which issubstantially equal to 2√Z₁ Z₂ , and that the resistances of said secondand third bridging devices are substantially zero.
 5. The powerdivider/combiner according to claim 2, wherein:the impedance of saidthird transmission line is raised by etching ground plane from undersaid third transmission line.
 6. The power divider/combiner according toclaim 2, wherein:said third transmission line is placed so close to saidfirst transmission line that interaction between said first transmissionline and said third transmission line in use will raise the impedance ofsaid third transmission line.
 7. The power divider/combirier accordingto claim 1, wherein:said power divider/combiner is a folded structurehaving both convex and concave curves in at least two of saidtransmission lines.
 8. The power divider/combiner according to claim 1,wherein:said power divider/combiner is a folded structure having bothconvex and concave curves in all of said transmission lines.
 9. Thepower divider/combiner according to claim 1, wherein:said powerdivider/combiner is a folded structure in which said transmission linesare not situated in a same plane.
 10. The power divider/combineraccording to claim 1, wherein: said power divider/combiner isimplemented as a stripline placed on a surface of a circuit board.